Method and device for sequencing signals from multiusers

ABSTRACT

Method for ordering a number K of given users in a digital signal equalizing and decoding device receiving the signals from the K users comprising at least the following steps:
         a step in which the K users, or at least the majority of K users, are ordered according to a defined criterion for a user k taking account of the power of user k corrected for intersymbol interference associated with this user k and with other users, and   an equalizing and decoding step.       

     Use of the method in a CDMA context.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is that of the transmission and broadcastingof digital signals, in particular in the presence of transmission noise.

The invention relates to a method for ordering the users at a receivercomprising an equalizer and a decoder before carrying out the signalprocessing.

The invention applies in a CDMA (code division multiple access) contextwith channel coding.

It applies in particular to third generation mobile radio systems.

2. Description of Related Art

In digital transmission, a receiver can be viewed as a succession ofseveral elementary functions each performing a specific process such asfiltering, demodulating, equalizing, decoding, etc.

The CDMA technique is a multiple access technique which will form thebasis of third-generation mobile radio systems.

The technique is based on the spread spectrum principle in whichtransmission takes place at a much higher bit rate than necessary foreach user, by multiplying the useful symbols by sequences of high bitrate symbols, called “spreading sequences”.

All the transmissions are then performed at the same frequency and atthe same times, separation between the users being obtained by differentspreading sequences.

The prior art describes receivers which make use of successiveinterference subtractions so as to order the users according to specificcriteria.

For example, one method consists in classifying the users according onlyto the power associated with a user.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a novel method making it possible inparticular to order the K users according to a criterion which takesinto account the power associated with a user but corrected for thecontribution of intersymbol interference associated with this user andalso with other users.

The invention also relates to a method and to a receiver where the usersare treated one after another, a demodulation step being followed by anerror decoding step.

The invention relates to a method for ordering a number K of given usersin a digital signal equalizing and decoding device receiving the signalsfrom the K users.

It is characterized in that it comprises at least the following steps:

-   -   a step in which the K users, or at least the majority of K        users, are ordered according to a defined criterion for a user k        taking account of the power of user k corrected for intersymbol        interference associated with this user k and with other users,        and    -   an equalizing and decoding step.

The criterion for ordering the users is defined, for example, in thefollowing manner:

$C_{k} = \underset{n = 1}{\overset{N}{\sum\;}}\;( {{h_{k,n}^{\dagger}h_{k,n}} - {\sum\limits_{m \neq n}\;{{h_{k,n}^{\dagger}h_{k,m}}}} - {\sum\limits_{j \neq k}\;{\sum\limits_{m}\;{{h_{k,n}^{\dagger}h_{j,m}}}}}} )$where h_(k1) denotes the lth column of a matrix H_(k) constructed fromthe vector containing the samples of the signal from user k, and ndenotes the temporal index of the coded symbol.

The equalizing and decoding step may comprise at least the followingsteps:

-   a) at iteration 1, for the user of index 1, to transmit the signal    to be demodulated to an equalizer of rank 1 then to a decoder of    rank 1 so as to obtain information from the estimated modulated    symbols from at least one of the decoders of rank (k−1), and-   b) for users k of index different to 1, to transmit the signal to be    demodulated to an equalizer of rank k and the various estimated    modulated symbols from at least one of the decoders of rank (k−1).

The invention also relates to a device for putting the signals receivedfrom several users into a given order before the signals are processedin a signal decoding device. It is characterized in that it comprises atleast a device suitable for determining a criterion for ordering thesignals from users, the criterion taking into account the power of agiven user k and of intersymbol interference for user k himself and forthe other users.

The device may comprise K modules, each module having at least oneequalizer linked to a decoder, and an equalizer of index k linked toseveral decoders of lower index 1 to k−1.

An equalizer of a module comprises, for example, at least a first blockwhich receives at least the signal to be demodulated, from user k, andthe estimates of the symbols associated with users 1 to k−1 and a secondblock designed to subtract the contribution from the past symbolsalready demodulated.

The method and device according to the invention are applied for exampleto demodulate a signal in the context of a space division and/or aCDMA-type code division multiple access scheme.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other advantages and features of the invention will become more apparenton reading the description provided for illustration purposes and not atall limiting, in conjunction with the figures in which:

FIG. 1 represents a coding chain,

FIG. 2 is a general scheme of a signal processing receiver,

FIG. 3 is a matrix allowing the received signal to be modeled,

FIG. 4 shows a specific structure of a receiver according to theinvention,

FIG. 5 shows an example structure of the equalizer used in the methodaccording to the invention, and

FIG. 6 is an alternative implementation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a system for coding the signals transmitted by K users whoshare the same propagation channel and who use waveforms spreadspectrally via codes, each user denoted by k, where k varies from 1 toK, having his own code. The users are considered as asynchronous userswho access the same frequency-selective propagation channel in order totransmit information.

The digital signal associated with user k is coded using a correctingcode 1 before being transmitted to an interleaver 2. These twofunctional blocks may be identical or very different for each user k.The correcting code is for example a convolutional-type code, but notnecessarily so.

The coded signal is then “spread” using a spreading sequence 3 _(k)—thepurpose of the index k is to denote the specific spreading sequence foreach user k. The different spreading sequences for each user thusprovide for discrimination between them. A modulated signal will see adifferent propagation channel 4 _(k) for each user. This typicallycorresponds for example to the uplink of a cellular mobile radio system.The concept of propagation channel includes for example any time shiftsresulting from an absence of synchronization between the K users.

The receiver sees the sum of the various contributions of the signalsfrom the various propagation channels. Thermal noise associated with theinput stages of the receiver described below or even interference fromsignals transmitted in the same frequency band, for example mobileterminals of neighboring cells using the same frequency, may be added tothese signals.

To simplify the description, the modulation operations, typically theshaping by a half-Nyquist filter and the transmission via carrier, whichare known to a person skilled in the art, are not represented in FIG. 1.

FIG. 2 shows a very general block diagram of a receiver.

The signal made of all the symbols of all K users arrives in a multiuserequalizer 6 before being transmitted to a decoding device 7 comprisingone or more decoders 7 _(k).

The equalizer 6 receives the signal from the various users along withinformation provided by the decoding block 7 on the coded symbols via alink 8, except when no decoding has been performed. The equalizer 6delivers as output weighted information on the coded symbols which makeuse of these two types of information, typically the probabilities oftransmission of the various possible symbols.

The functions for deinterleaving and shaping the weighted informationbetween the equalizer and the decoders are not represented in thefigure. These functions are for example for translating information onthe 8-ary symbols into probabilities on the bits when the modulation isin 8 states and the code is binary.

The decoder or decoders 7 _(k) receive information from the equalizerand make use of the information associated with the error-correctingcode so as to provide more reliable information on the useful symbolsand therefore the coded symbols. For example for a convolutional code,the decoder can be of the “MAP” (maximum a posteriori) type andcalculates the probabilities on the useful symbols from the knowledge ofthe probabilities on the coded symbols.

The weighted information is then reinterleaved and reshaped before beingreinputted into the “equalizer” block. The data interleaving step iscarried out according to a method known to a person skilled in the artand will not be detailed in this description. One way of proceedingconsists for example in writing the data column by column in a matrix ofappropriate dimension and reading this matrix row by row for example.This advantageously results in any errors being distributed during thereading stage of the signal. The shaping is performed according toconventional methods which are known to a person skilled in the art, andhence are not detailed.

The signal received by the equalizer can be modeled as described below.

It is assumed that a block of received samples can be arranged in avectorial form Z. This block of samples arises from the contribution ofthe K users, each of them contributing with N coded symbols.

The contribution from an arbitrary user of index k to the receivedsignal is obtained, without taking into account the modulationoperations on the carrier, via the following operations:

-   -   spreading by a factor Q of each coded symbol,    -   shaping, for example by a half-Nyquist filter,    -   filtering by the propagation channel by introducing multiple        paths for example,    -   anti-aliasing filtering at the receive end, and    -   sampling.

The signal from user k before sampling is given for example by equation(1) below

$\begin{matrix}{{z_{k}(t)} = {\underset{n}{\sum\;}\;{a_{n,k}{\sum\limits_{q = 1}^{Q}\;{{c_{q,k} \cdot ( {h \otimes p_{k} \otimes f} )}( {t - {qT}_{c} - {nT}_{s}} )}}}}} & (1)\end{matrix}$

In this equation, h represents the shaping filter at the transmissionstage, p_(k)(t) is the propagation channel specific to a user k, f(t) isthe receive filter before sampling, T_(c) is the chip period, inverse ofthe modulation rate, and T_(s) is the symbol period before spreading.The symbols a_(n,k) are the coded symbols of the user k and the symbolsc_(q,k) are the chips of the spreading sequence, where n corresponds tothe temporal index of the coded symbol and q corresponds to the index ofthe chips or symbols of the spreading sequence.

The symbol

{circle around (×)}

represents the convolution.

Suppose:

$\begin{matrix}{{{s_{k}(t)} = {\sum\limits_{q = 1}^{Q}\;{{c_{q,k} \cdot ( {h \otimes p_{k} \otimes f} )}( {t - {qT}_{c}} )}}}\text{Then:}} & (2) \\{{z_{k}(t)} = {\sum\limits_{n}\;{a_{n,k}{s_{k}( {t - {nT}_{s}} )}}}} & (3)\end{matrix}$

Equation (3) shows that the contribution of each user k can be put inthe form of filtering of a train of symbols by a certain functions_(k)(t) which contains the effects of the spreading, of the shapingfilter at the transmission stage, of the propagation and of the receivefilters before sampling.

Let S_(k) be the vector containing the samples of s_(k)(t). Then thesampled signal can be rewritten for the user k as follows:Z _(k) =H _(k) ·A _(k)  (4)where A_(k) is the vector of N symbols a_(n,k) and H_(k) is a matrixconstructed from the vector Sk in the manner described in FIG. 4.

The columns of the matrix H_(k) are constructed from shifted versions ofthe sequence S_(k), and each column corresponds to a new symbol of thevector A_(k) and the shift corresponds to the number of samples persymbol. The recovery between the various shifted versions of S_(k)corresponds to the duration of the overall impulse response (filtersplus propagation channel).

The complete model can then be represented as follows:

$\begin{matrix}{Z = {{\sum\limits_{k = 1}^{K}\;{H_{k}A_{k}}} + W}} & (5)\end{matrix}$

In this equation (5), W is a noise vector associated with theinterference outside the cell and with the noise inside the receiver.

Criterion for Ordering the Users Before Signal Processing

The different equalization and decoding steps can advantageously beapplied to a set of users who are ordered according to a criteriontaking account of the power associated with a user, from which thecontribution of intersymbol interference for this same user and for theother users is subtracted.

This criterion is determined for example as follows:

-   -   h_(k1) denotes the lth column of the matrix H_(k) described in        relation to FIG. 3. This vector denotes the overall impulse        response of the channel for the 1th symbol of the block to be        demodulated, of user k,    -   the criterion C_(k) is determined as follows

$C_{k} = \underset{n = 1}{\overset{N}{\sum\;}}\;( {{h_{k,n}^{\dagger}h_{k,n}} - {\sum\limits_{m \neq n}\;{{h_{k,n}^{\dagger}h_{k,m}}}} - {\sum\limits_{j \neq k}\;{\sum\limits_{m}\;{{h_{k,n}^{\dagger}h_{j,m}}}}}} )$where the symbol † denotes the transposed conjugate, the first termcorresponds to the power associated with user k, the second termcorresponds to the contribution from the user and the third term to thecontribution to intersymbol interference for all the other users.

This calculation involves correlations between the signals correspondingto the various symbols whether they are transmitted by the user khimself or by the others.

In fact, the method measures the existing distance, at least, betweentwo opposed symbols at the output of a filter designed according to theimpulse response h_(kn), whatever the values of the other symbols whichinterfere.

The users are arranged in a decreasing order before performing the stepsof the method according to the invention.

FIG. 4 shows a structure of a receiver 10 according to the inventioncomprising several equalizers 11 _(k) and several decoders 12 _(k),where the index k is used to identify a user. The receiver thereforecomprises as many module as there are users, each module being formedfrom an equalizer and a decoder.

The signal comprising the symbols from all the users is received by eachequalizer 11 _(k) of the receiver 10.

Step a)

The equalizer of rank 1 referenced 11 ₁ receives for example the samplesfrom the signal to be decoded, without information a priori on its ownsymbols or on the symbols of the other users. The information resultingfrom this first equalization is transmitted to the decoder of rank 1denoted by 12 ₁ which provides more reliable information on the usefulsymbols and therefore the modulated symbols of the user.

Step b)

The reliable information, on the useful symbols, obtained for user 1 isthen transmitted to the equalizer of rank 2, 11 ₂, which also receivesthe samples of the received signal to be decoded. The equalizer thusenables the interference associated with the user of rank 1 to be takeninto account, while still knowing nothing a priori about the symbols ofthe users of rank greater than or equal to 2. The information from thissecond equalization is then transmitted to the decoder of rank 2, 12 ₂,which will provide information on the useful symbols of the user of rank2.

Step b) is executed as many times as there are users of rank differentto 1, that is K−1 times.

To generalize, for the user of index k, the samples of the signal aretransmitted to the equalizer of rank k, 11 _(k), which also receives theuseful symbols on the users of rank 1 to k−1, resulting from thedifferent decoders 12 ₁, . . . , 12 _(k−1). The equalizer thus allowsaccount to be taken of the interference associated with the users ofrank 1 to (k−1) for decoding the signal associated with user k. Theinformation from this kth equalization is then transmitted to thedecoder of rank k, 12 _(k), which will provide information on the usefulsymbols of the user of rank k.

The last equalizer has information on all the other users.

According to an embodiment of the invention, the equalizer used has atwo-part structure. The first part is for subtracting for a user k inquestion the participation of the users of rank 1 to k−1, and the secondpart corresponds to a decision feedback equalizer (DFE) structure havingthe characteristics described below.

FIG. 5 shows the structure of an equalizer operating by using thefeedback principle described previously in relation to FIG. 4.

During equalization of user k, the purpose of the first block 20 is tosubtract from the received signal Z the contributions of the users ofindices between 1 and k−1, that is users already processed. To do this,it receives the vectors of estimated symbols corresponding to theseusers. The estimate of the symbols is made for example by calculatingthe mathematical expectation of the value of the symbol from theprobabilities obtained during the decoding steps.

The purpose of the second block 21 is to process the intersymbolinterference corresponding to the symbols of user k himself, and thenoise resulting from the users of indices k+1 to K not yet processed andfrom the thermal noise. The equalizer has a DFE structure, that is it ismade up of a “transverse filter” part and a “decision in feedback” part.The “transverse filter” part is calculated by taking into account theknowledge of the structure of the noise generated by the users not yetprocessed and by the thermal noise. The “decision in feedback” part hasa sequential operation. For each index n symbol of the current user, itsubtracts the contribution from the already decided lower index symbols.

The second block according to the invention comprises for example atransverse filter 22 such as a matrix being applied to the vector U, andthe resulting vector V is sampled at the symbol rate. The coordinates v₁of this vector are then passed into a loop comprising a decision unit 23and a recursive filter 24. The decision unit is for obtaining a decisionon the symbols. At each new symbol, the recursive filter 24 subtractsthe contribution from the symbols already decided in the block andforwarded. The loop also comprises a weighted output 25, before thedecision unit, for transmitting the symbols to the decoder.

The calculation from the transverse filter 22 and the recursive filter24 is carried out for example in the manner described below.

The calculation of the filters uses as criterion the minimization of themean square error between the weighted output of the equalizer and thevector of symbols of user k.min(E(∥Y_(k)−A_(k)∥²))  (6)

From equation (5), the vector U can be represented as follows

$\begin{matrix}\begin{matrix}{U = {{H_{k}A_{k}} + B}} \\{B = {{\sum\limits_{l = {k + 1}}^{K}\;{H_{l}A_{l}}} + W}}\end{matrix} & (7)\end{matrix}$

In this equation (7), the noise is made up of signals transmitted by therank k+1 to K users not yet decoded and of the initial additive noise W.

For a white noise W, the expressions for the filters are given below:

The correlation matrix for B is

$\begin{matrix}{R_{B} = {{\sigma_{A}^{2} \cdot {\sum\limits_{l = {k + 1}}^{K}\;{H_{l}H_{l}^{\dagger}}}} + {\sigma_{W}^{2} \cdot {Id}}}} & (8)\end{matrix}$where σ_(A) ² is the power of the modulation symbols, σ_(w) ² is thepower of the noise and Id is the identity matrix

-   -   the matrix Q is defined by

$\begin{matrix}{Q = {{H_{k}^{\dagger}R_{B}^{- 1}H_{k}} + {\frac{1}{\sigma_{W}^{2}}{Id}}}} & (9)\end{matrix}$

-   -   the Cholesky composition of this matrix is formed as follows        Q=(ΣL)†(ΣL)  (10)

In equation (10), Σ is a diagonal matrix and L is a lower triangularmatrix.

The expression of the transverse filter which minimizes criterion (6) isT=Σ ⁻² L ^(−1†) H _(k) †R _(B) ⁻¹  (11)

The recursive filter is determined by the rows of (L-Id).

According to an embodiment of the invention, the device uses aninterference canceler when all the symbols to be demodulated have beenthe subject of at least one decoding stage. In this situation, unlikethe decision feedback equalizer, when it is desired to decide the symbolj of the user k, all the other symbols of the same user or of the otherusers can be considered known, even if in practice only the estimates ofthese symbols are known. It is then possible to subtract theircontribution totally.

FIG. 6 shows the principle of this canceler in block diagram form.

Compared to the decision feedback equalizer case in FIG. 5, there ismore information on all the symbols from iteration i−1 for certainusers, for example those of index 1 to k−1, and from the currentiteration i for the other users of index k+1 to K.

The principle implemented consists in:

-   -   subtracting the contributions from all the users other than user        k being processed, by using the most recent information for each        of them, for the users of index 1 to k−1 the estimates of the        vectors of the symbols during iteration i and for the users of        index k+1 to K the information of the estimates obtained during        iteration (i−1), this iteration being carried out in the first        block 20 which generates a vector U,    -   filtering the received samples for the user k in the filter T        22, and the resulting vector V is then sampled for example at        the symbol rate,    -   subtracting in the second block 21, for a given symbol n, the        residual intersymbol interference coming from the other symbols        of the user k. The contribution from the symbols 1 to n−1 is        determined by the filter P_(n), 26 and the contribution from the        index n+1 to N symbols is determined by a filter Q_(n), 27.

In this way, compared with the decision feedback, much richerinformation is available than the previous decided symbols, that isinformation on the past but also future symbols from the previousiteration.

After the processing steps, the next step is the decoding step, and thenthe next user is considered and the contribution of user k is subtractedby using the results obtained during iteration i and no longer i−1.

1. A method for ordering a number K of users in a digital signalequalizing and decoding device receiving signals from the K users,comprising: ordering, for a user k, at least a majority of the K usersaccording to a defined criterion, said defined criterion comprising thepower of user k, from which intersymbol interference from said user kand from other users is subtracted out, equalizing the ordered number Kof users; and decoding the signals of the number K of users, wherein thedefined criterion involves correlations between symbols transmitted bythe user k and symbols transmitted by the other users.
 2. The method asclaimed in claim 1, wherein the defined criterion for ordering the usersis determined as follows:$C_{k} = \underset{n = 1}{\overset{N}{\sum\;}}\;( {{h_{k,n}^{\dagger}h_{k,n}} - {\sum\limits_{m \neq n}\;{{h_{k,n}^{\dagger}h_{k,m}}}} - {\sum\limits_{j \neq k}\;{\sum\limits_{m}\;{{h_{k,n}^{\dagger}h_{j,m}}}}}} )$where h_(k1) denotes a lth column of a matrix H_(k) constructed from avector including samples of a signal of user k, and n denotes a temporalindex of a coded symbol.
 3. The method as claimed in claim 1, whereinthe equalizing and decoding comprises: (a) at iteration 1, for a user ofindex 1 to transmit a signal to be demodulated to an equalizer of rank1, and then to a decoder of rank 1, to obtain information from estimatedmodulated symbols from at least one of decoders of rank (k−1); and (b)for users k of an index different from 1, to transmit the signal to bedemodulated to an equalizer of rank k and various estimated modulatedsymbols from at least one of the decoders of rank (k−1).
 4. The methodas claimed in claim 3, further comprising plural iterations and wherein,for an iteration different from the first iteration, step b) comprises,during an ith iteration, transmitting to a first block of the equalizerof rank k the symbols of user k to be demodulated, estimates of thesymbols of users 1 to k−1 obtained during the ith iteration andestimates of the symbols of users k+1 to K obtained during an (i−1)thiteration and the estimates of the symbols of user k from the (i−1)thiteration.
 5. The use of the method as claimed in claim 1 to demodulatea signal in a context of at least one of a space division and aCDMA-type code division multiple access scheme.
 6. A device for puttingsignals received from plural users into a given order before the signalsare processed in a signal decoding device, comprising: a deviceconfigured to determine a criterion for ordering the signals from theplural users, the criterion comprising a power of a given user k, fromwhich intersymbol interference from user k and from the other users issubtracted out; and K modules, each module having at least one equalizerlinked to a decoder, and wherein an equalizer of index k is linked toplural decoders of lower index 1 to k−1.
 7. The device as claimed inclaim 6, wherein the equalizer comprises at least first and secondblocks, the first block receives at least the signal to be demodulated,from user k, and estimates of symbols associated with users 1 to k−1,and the second block is configured to subtract a contribution from pastsymbols already demodulated.
 8. The device as claimed in claim 7,wherein the first block receives, at an ith iteration, at least thesignal from user k to be demodulated, at least estimates of symbolsassociated with users 1 to k−1 corresponding to the ith iteration andestimates of symbols of users k+1 to K obtained at an (i−1)th iteration,and the second block receives the estimates of the symbols of user kobtained at the (i−1)th iteration.
 9. The use of the device as claimedin claim 6 to demodulate a signal in a context of at least one of aspace division and a CDMA-type code division multiple access scheme.